The present invention relates to extruded closed-cell polypropylene foam. The present invention also relates to several methods for making this extruded closed-cell polypropylene foam.
The polypropylene foam of the present invention has uniform and fine closed cell structure.
Since polypropylene generally has a high crystallinity and a low melt viscosity and is difficult to cross-link, it has hitherto been quite difficult to obtain a highly expanded product therefrom. One such kind of product is radiation-crosslinked foam-sheet.
However, since polypropylene is superior to polyethylene in heat resistance, polypropylene is expected to become useful in the field requiring a heat resistance of 100 degrees centigrade or higher, such as heat-insulating material for hot water piping, if a highly expanded product can be obtained therefrom with relative inexpensiveness by the non-crosslinking extrusion foaming process similarly to polyethylene.
Also, the foamed closed-cell polypropylene products of the present invention have good cushioning properties at low densities.
Closed-cell polypropylene foams, like polyethylene foams, could be widely employed in the field of packaging in order to provide cushion properties. Fragile objects intended to be transported may be encased and supported in a closed cell foamed polypropylene cushion adapted to conform to the external shape of the object for which protection is desired. One consideration when providing cushioning is shipping costs. Larger thicknesses of cushioning foam result in excess packaging sizes and concomitant elevated shipping costs. So it is desirable to reduce density of the cushioning material to a minimum while retaining the necessary cushioning properties.
In U.S. Pat. No. 4,522,955 (GB 2,099,434) there is disclosed an extrusion process for highly expanded polypropylene resin foams. These specific polypropylene resin foams use a polypropylene resin having at a stated temperature a minimum value of melt tension and a stated upper limit for a maximum/minimum ratio of melt tension. More specifically, for this non-crosslinking extrusion process, in addition to the ingredients of polypropylene resin and volatile foaming agent, it is required that a nucleus-forming agent, such as inorganic substances, carbon dioxide generators, inorganic or organic acids and thermal type decomposition type chemical foaming agents, be included as part of the mixture to form the polypropylene resin foams of this disclosure.
The present invention is directed to extruded closed-cell polypropylene resin foams, produced with and without a nucleus-forming agent.
The present invention is also directed to extrusion methods of making these closed-cell polypropylene resin foams with and without a nucleus-forming agent.